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Where on earth is Dark Matter?

  1. Aug 4, 2014 #1
    Dark Matter constitutes 26.8% (wikipedia) of the observable universe.

    So where is it?

    I ask this question because although it is said to be not visible. It must interact with light matter at a gravitational level.

    It only makes sense to me that dark matter must be all around us and throughout us. Why would dark matter avoid light matter but still interact with it? (antisocial?)

    Therefore if dark matter does interact with light matter (gravitationally) and therefore is contained in and around us then how did we find it?
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  3. Aug 4, 2014 #2


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    The search for dark matter is on-going. It IS all around us to some extent, although rather trivially so, and the amount is so small that the gravitational effect on Earth and Earth-bound objects is probably about the same as a rounding error in the 20th significant digit. Unmeasurable.
  4. Aug 5, 2014 #3


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    Normal matter experiences friction. It is through that friction that large gas clouds collapse to form dense objects, such as planets and stars.

    Dark matter doesn't experience hardly any friction. So it remains almost exactly as spread-out as it started.

    So you can think of a galaxy like our own as a giant, smooth blob of dark matter, with a small but very dense group of stars at the center. That small blob of stars is what we can visibly see of the galaxy.

    So the answer is: it's all around us. But because it can't collapse into dense objects, there's just not very much around our planet. There is certainly some, however, which is why many physicists are currently attempting to detect the rare collisions of dark matter with normal matter.
  5. Aug 5, 2014 #4
    Is that the consensus?
    How and to what extent does friction contribute to the formation of denser objects?

    This is new to me so it certainly puts a spanner in my works.

    Your also implying that dark matter doesn't even or only slightly interacts with itself.

    My assumption was that after the big bang light matter and dark matter would be spread rather evenly around the universe. Surely as dark matter constituted a vastly greater amount of matter it would have started coalescing first and then drawn light matter in with it. It would make sense to me that we are more dark matter than light matter.

    Of course if friction plays a big role on the coalescing of matter, then my assumption is off. but even while typing this I cannot get my head around friction being the driver to condensation of large objects rather than simply a side effect.
  6. Aug 5, 2014 #5


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    Think about "collisions" instead of friction. If you shoot a DM particle through a cloud of other particles it will simply fly through. A normal particle has at least the chance to hit one of the particles in the cloud and lose some of its kinetic energy.
    An important point is also that in these collisions, you can generate EM radiation (light) which carries erergy away very efficiently. That makes it far easier for clouds to collect new particles and to cool down.
  7. Aug 5, 2014 #6


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    No friction, no clumping. Two particles that get really, really close to each other "hit" each other and slow down and come back and do it again until eventually the friction has slowed them down enough that the join. Dark matter doesn't do that because it does not interact in that way.

    Yes, it does.

    No, he is not implying it, he is saying it outright.

    Yes, that's what is believe to have happened but dark matter formed clouds, never coalesced into solid objects. Gravitationally, it attracted regular matter ("light matter" as you are calling it) which DID form solid objects.

    See above.

    Yes, it is.

    Welcome to cosmology.
  8. Aug 5, 2014 #7

    So let me get this straight.

    Two particle fall into each others gravity influence and start to circle each other. At first they just follow long elliptical loops. Over time the loops start to get smaller and smaller dragged in by gravity. but in doing so the particles also accelerate conserving energy. Therefore they can never meet.

    That's why we need collisions to drop the energy down enough to allow them to finally make contact (and form a molecule?). This works for light matter.

    Dark matter on the other hand fails to interact enough to lose any energy and therefore the dance never ends. They just spin off in a seething invisible cloud. but the sum of their gravity still has a large influence on galactic scales.

    Does that about sum it up?
  9. Aug 5, 2014 #8


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    Friction is pretty much the entire picture.

    Think of it this way, if you have a large, diffuse cloud of gas, that gas has a lot of gravitational potential energy. For that cloud to collapse into a compact object like a star, it has to lose a large fraction of that energy. If it doesn't lose the energy, it doesn't collapse.

    The exact processes are somewhat complicated, but "friction" remains an accurate description.
  10. Aug 6, 2014 #9


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    It's even worse, they would have too much energy to even start circling each other. They'd just fly off. If they manage to start circling each other, their orbits would be stable, not getting smaller and smaller.
    Yes, at least if they are electrically neutral. Macroscopically, if you add elastic collisions, you generate friction. If you add inelastic collisions, you also have the possibilty to radiate away heat. Both is important for the formation of high density objects.
  11. Aug 6, 2014 #10


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    Chris, this is a good discussion and it is going somewhere, but nobody has mentioned "three body gravitational interactions" (let's abbreviate that "3BGI") yet. That's an important way for DM to shed excess energy so that DM clouds can contract. Because DM can't do the collision heating radiation trick that OM uses to shed energy. First I will recap what you already got thru, which was good:
    So OM clouds condense primarily by collisional heat being radiated away---energy leaves the cloud that way---so the OM cloud can gradually contract.

    But DM cannot radiate heat. So how can a DM cloud shed energy?

    Do you understand the "slingshot" maneuver used to let space probes going to the outer planets pick up extra orbital speed by passing close by Earth and other massive bodies? Orbiting bodies can EXCHANGE energy. The probe whips by Mars on its way out to Jupiter. it picks up a little speed and is flung by Mars a little faster. Mars is so massive we don't notice that it was slowed down by this interaction, but actually Mars loses the energy which the probe gains. Mars is infinitesimally slowed down by the slingshot encounter, which speeds the probe up.

    Space missions use this trick. it is standard. It is called "three body" because the two principals (the probe and the planet it steals energy from) are both orbiting the Sun, so there are actually 3 main players.

    Maybe we should make up a term like "N body gravitational interaction" (NBGI) for the same thing where there is no one special designated central body like the sun. Just a collective gravitational field of the cloud of particles and they are all orbiting each other within the cloud.
    And then A whips past B in such a way that it slows B down and A picks up so much extra energy that gets ESCAPE VELOCITY! And it is totally flung out of the cloud and so A carries away some of the cloud's energy and this allows the remaining particles to condense a little.

    This is a way for a cloud of particles to gradually condense (much less effectively than if they could radiate collision heat.) A poor substitute, but still it works well enough to enable DM to gather into largish clouds.

    There is more to the story (what happens to the ejected particles that carry away the unwanted energy and angular momentum.) Expansion, over long distances, plays a part. The expatriates can eventually meet and be taken in by other clouds. That's another chapter. The main thing is DM clouds CAN condense somewhat, they just aren't as good at it as OM clouds.
  12. Aug 7, 2014 #11

    So you are saying they work like perpetual motion machines?

    No No don't frown I am only joking. I thought it was very funny when someone tried to tell me the slingshot maneuver was proof of perpetual motion. (they did not realise the target was slowed down. Its hard to imagine something as massive as the earth or other heavenly body slowed down by a tiny rocket).

    In answer to your question I do understand the slingshot. everything said so far makes great sense and now I am going on the talk circuit to talk about Dark Matter!

    As is usual I come here and ask a dumb question and come away just that little bit more knowledgeable. So thanks guys I appreciate it.
  13. Oct 4, 2014 #12

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    That's very nearly right - with no reason to stick together, the infalling objects would just fly past each other and cycle back. This is perpetual motion - but not a perpetual motion machine since you cannot extract useful work from the system in perpetuity.

    But pay close attention to marcus' reply above - it fills in some important gaps in what the others have been telling you and should shed some light on your other question:

    "friction" in the above discussion is a handy shorthand for dissipative processes - the main one used by OM is radiation. DM does not radiate - that is why it's dark - therefore it has trouble losing energy, therefore low friction and not very clumpy. Bits clump when they come together and stick ... this is inelastic, so what happens to the excess energy? It has to be send off with a third bit ... that can be light (radiation) or by getting another bit of matter of some kind to take it away.

    So now you've got two bits of dark matter to hang about together (by sending a third bit off with the excess energy) ... is that a stable situation? Do they have any other reason to stay together so a small bump does not send them apart again?

    Low friction does not mean zero friction:
    See Debatista and Sellwood: Dynamical Friction and the Distribution of Dark Matter in Barred Galaxies
    http://arxiv.org/abs/astro-ph/9710039 and
    ... which seems apropos to your original question.

    Ardi, Tsuchiya and Burkert Constraints on the clumpiness of dark matter halos thorugh heating of disk galaxies
    ... should help with your questions about clumping.

    One thing to remember - we are still narrowing down the dark matter properties. "Dark matter" is the name given to whatever it is that we cannot see but has a gravitational effect. We expect very little clumpiness because we cannot see it (does not radiate) but we expect some clumpiness because it interacts gravitationally.
  14. Oct 4, 2014 #13
    I don't think this part of the OP question has been addressed. Apologies if I missed it.
    Galaxy rotation curves

    Unless there have been new findings that I'm not aware of, the existence of dark matter is still uncertain. The problem of galaxy rotation curves may have another, yet undiscovered explanation.
  15. Oct 5, 2014 #14
  16. Oct 6, 2014 #15


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    Neutrinos are already known to be one form of dark matter. They just do not appear to be sufficient to satisfy all the properties necessary to fully account for all the issues associated with DM. The elusive sterile neutrino remains a player. It is even more non-interactive than known, left handed neutrinos and sufficiently enduring to be primordial relics.
  17. Oct 6, 2014 #16
    I think the post is asking: how do we see dark matter?

    1. In the CMB:

    "The technical name for this is the power spectrum, since it’s a measurement of the rate of energy carried by the CMB photons as they arrive at WMAP. The three peaks are key: they will tell us exactly what the contents of the universe are!
    • The first peak contains information about the total amount of stuff in the universe: ordinary matter, dark matter, photons, neutrinos, dark energy, and anything else we might not yet know about. The size and location of the peak is related to the geometry of the universe: how “flat” it is. Note that this peak covers areas of the sky much larger than the temperature fluctuations we’re seeing (the Sun is 0.5° on the sky!), so it’s not produced by sound waves in the early universe, but by the total energy driving expansion.
    • The second peak is the sound waves, and tells us exactly how much ordinary matter there is in the universe.
    • The third peak takes into account both the ordinary and dark matter. Combining this with the second peak therefore gives us the amount of dark matter.
    So now we have it: by taking the three peaks together, we have the total amount of matter, the total amount of ordinary matter, and all the stuff together. Combining this data in different ways gives us the amount of dark matter (peaks 2 and 3) and dark energy (peaks 1 and 3). "

    [ http://galileospendulum.org/2012/02/17/the-genome-of-the-universe/ ; my bold]

    The reason the 3d peak sees both OM and DM is because the gas molecules that scatter the photons have had time to feel the DM effects, IIRC.

    2. By weak lensing:


    ... and many more observations.

    But remember, it is too diffuse to see easily:

    "So if there’s a sea of dark matter that permeates space where we are — all through the Solar System — the outer planets should see a slightly different (greater) mass than the inner planets. And if there’s enough dark matter, it should be detectable.

    “Let’s calculate it,” the professor said to me, and so we spent the next half-hour doing just that. When we finished, we’d found that about 1013 kg of dark matter ought to be felt by Earth’s orbit, while around 1017 kg would be felt by a planet like Neptune. These values are tiny; the Sun has a mass of 2 ✕ 1030 kg, while values in the 1013 - 1017 kg range are the mass of a single modest asteroid. Someday, we may understand the Solar System well enough that such tiny differences will be detectable, but we’re a good factor of 100,000+ away from that right now."

    [ http://scienceblogs.com/startswitha...matter-affect-the-motion-of-the-solar-system/ ; my bold]
    Last edited: Oct 6, 2014
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